As is well known, there are vast quantities of heavy aromatic by-products produced from the steam cracking of gas oil or naphtha from the catalytic cracking of hydrocarbons and from high temperature coke production from coal. In general, these heavy aromatic by-products are composed of alkyl substituted polynuclear aromatic compounds. The heavy aromatic fractions, of course, are not uniform, but contain a complex mixture of polynuclear aromatic oils, asphaltenes and, of course, the usual quantities of impurities. These heavy aromatic by-products also vary significantly in their chemical structure and molecular weight distribution, aromatic ring distribution and coking characteristics. By coking characteristics, of course, is meant their tendency to form isotropic coke on heating to temperatures in the range of about 400.degree. C. to about 550.degree. C. Notwithstanding these differences, the just mentioned heavy aromatic feedstocks are used for production of pitches which have various varying microstructures (i.e., isotropic or anisotropic).
It is believed that the major portion of the heavy aromatic feedstock which is essential for the production of anisotropic pitch is the low molecular weight polynuclear aromatics present in the heavy aromatic feedstocks, i.e., with the polynuclear aromatics having aromatic rings of from about 3 to 7. These multi-ring polynuclear aromatic molecules on thermal treatment at high temperatures, for example on temperatures in the range from about 400.degree. C. to about 500.degree. C., undergoes several reactions, such as dealkylation, ring condensation, dimerization, trimerization and polymerization, resulting in the production of a highly aromatic pitch in which the molecules tend to align themselves in such a manner that when a polished sample of the pitch is viewed in the plane of polarized light seem to have a highly anisotropic or crystalline type of structure. Further, carbonization of such pitches tend to yield highly ordered structures which are most important in the manufacture of carbon artifacts and particularly carbon fiber and needle coke.
As indicated, however, these heavy aromatic feedstocks are complex mixtures, and, as a consequence thereof, contain significant quantities of other materials which when heated at elevated temperatures, for example, in the range of 400.degree. C. to 500.degree. C., result in the generation of isotropic material, such as isotropic coke which is not particularly desirable for carbon artifact manufacture, and particularly is considerably undesirable in the manufacture of carbon fibers since the presence of coke particles or, indeed, the presence of other high molecular weight components present in the resultant pitch are detrimental to spinning the pitch into fibers for subsequent carbonization. Indeed, coke particles are even believed to be detrimental to product quality and generally are responsible for breaks in the fibers, plugging of the spinneret and numerous other difficulties are associated with the presence of such quinoline insoluble substances.
To summarize some of the requirements then for a feedstock material suitable for carbon artifact manufacture, and in particular carbon fiber production, the first requirement is the ability of the feedstock to be converted to highly optically anisotropic material. Additionally, the highly optically anisotropic material should have a relatively low softening point so that they can be deformed and shaped into the desirable article. Insofar as carbon fiber manufacture is concerned, a suitable pitch which is capable of generating the requisite highly ordered structure also must exhibit sufficient viscosity for spinning. As eluded to above, many carbonaceous pitches have relatively high softening points and, indeed, with many carbonaceous pitches incipient coking occurs frequently in such materials at temperatures where they have a sufficient viscosity for spinning.
Additionally, suitable feedstock should be substantially free of coke or other infusible materials and/or undesirably high softening point components and materials likely to generate such infusible materials that are undesirably high softening point components prior to the spinning temperatures of the pitch.
Last, but not least, a suitable feedstock for carbon artifact manufacture should be able to be converted to a suitably high optically anisotropic material at a reasonable rate. For example, in U.S. Pat. No. 3,919,376, it is disclosed that 350.degree. C. is the minimum temperature generally required to produce optically anisotropic material, mesophase, from a carbonaceous pitch. More importantly, however, is the fact that at least one week of heating is necessary to produce a mesophase content of about 40% at that minimum temperature. The mesophase, of course, can be generated in shorter times by heating at higher temperatures. However, at temperatures in excess of about 425.degree. C. incipient coking and other undesirable side reactions do take place which can be detrimental to the ultimate product quality.
One component which is present in heavy aromatic feedstocks and which is detrimental to the production of a carbonaceous pitch suitable for carbon artifact manufacture is asphaltene. As is well known, asphaltenes are solids which are insoluble in paraffinic solvents and have high melting points, and most importantly asphaltenes tend to form isotropic coke readily because of their highly aromatic ring structure and high molecular weight. Indeed, the coking characteristics of asphaltenes can be determined by the standard analytical test used in the carbon industry (SMTTP Method No. TT-10-67). Basically in this test, a sample of asphaltene is carbonized at 550.degree. C. for 2 hours and the resulting coke generated is determined quantitatively.
The deasphaltenation of the heavy aromatics, as is well known, is achieved by solvent extraction of the feed using typically paraffinic solvents having from 5 to 7 carbon atoms. Such a technique, however, has not been successful in deasphaltenating cat cracker bottoms to the extent that the cat cracker bottom is converted into a feedstock suitable in carbon artifact manufacture.